"Pushing rope" is a slang term for penetration with a penis that is not erect. It's a metaphor that compares a soft penis to a piece of rope, which are equally difficult to insert and equally not likely to satisfy a partner.
More About Pushing Rope
Although some approaches to sexuality actually encourage penetration with a soft penis, it is usually a frustrating experience to try to have penetrative sex with one. "Pushing rope" is a negative slang term to describe this situation.
More famously, the term "pushing rope" has been used in the adult animated show Archer, where Pam complains that Archer is "pushing rope" while they're having sex, also intimating the fact that while the sex has been amazing for him, it certainly is not for her.
